docs: def id_field_name(cls):
          """
          If only one primary_key, then return it. Otherwise, raise ValueError.
          """
          if cls._cache_id_field_name is None:
              pk_names = cls.pk_names()
              if len(pk_names) == 1:
                  cls._cache_id_field_name = pk_names[0]
              else:  # pragma: no cover
                  raise ValueError(
                      "{classname} has more than 1 primary key!"
                      .format(classname=cls.__name__)
                  )
          return cls._cache_id_field_name